How does Jerry know that Barsad is lying about Roger Cly’s death (From Book 3 Chapters 6-10)

Jerry Cruncher interjects, saying that he looked in that coffin, and there was no body in it. Jerry Cruncher was a grave robber, and he knew that the coffin was filled with stones.

"I tell you," said Jerry, "that you buried paving-stones and earth in that there coffin. Don't go and tell me that you buried Cly. It was a take in. Me and two more knows it."